1. Field of the Invention
The present invention relates to a mechanical gear hob machine having an automatic stock device to enable the hob to cut or re-cut gear parts that already have teeth by automatically synchronizing the workpiece teeth with the hob cutter.
2. Description of the Related Art
In the hobbing process, a rotating, generally cylindrical shaped tool having helically arranged cutting surfaces is brought into contact with the rotating workpiece, generally a gear blank. In spur gear hobbing, the tool and workpiece rotate in a timed relationship as though the workpiece were a gear rotating in mesh with a worm gear represented by the hob cutter. When helical gears are hobbed, the timed relationship is different. A supplemental rate of motion is applied to the workpiece, either advancing or retarding the workpiece rotation relative to the hob cutter vertical position, in order to develop the appropriate helix angle across the face width of the gear being machined. Hobbing is primarily used for producing spur and helical gears, however, the hobbing process may also produce other products.
A typical mechanical gear hob rotates the hob cutter about a generally horizontal axis while the workpiece is rotated about a vertical axis. A main drive motor rotates both the hob cutter and the workpiece through a complex drive train in a synchronized motion so that the workpiece is rotated in phase with the hob cutter. In addition, the hob cutter and workpiece can move vertically relative to one another to cut across the entire face width of the workpiece.
A differential in the hobbing machine is used to take input from the vertical feed drive and add a portion of it to the drive train connecting the hob cutter spindle to the worktable. This device has been used to simplify the production of helical gears. For cutting helical gears, the timed relationship of the hob cutter to the workpiece is slightly advanced or retarded as the hob cutter moves vertically along the face width of the workpiece. For cutting spur gears, the differential is xe2x80x9clocked-outxe2x80x9d, which involves removing the differential change gears and holding the feed drive input to the differential in place, i.e.xe2x80x94no rotation is allowed. There is no input into the differential and the hob cutter forms a spur gear having straight teeth parallel to the gear axis.
Mechanical gear hobs cannot cut gears from parts that already have teeth, such as oversized parts or near-net gear forgings, without the operator first aligning the teeth of each gear to the teeth of the hob cutter before starting the machine. This manual stock divide procedure is slow and tedious and does not work with hobs that have automatic part loaders.
Automatic stock divide can be performed on a computer numerical controlled (CNC) gear hob machine. A CNC machine has separate drives for the hob cutter rotation, hob cutter shift, workpiece rotation, etc. A controller operates all of the various drives to coordinate the machine operation. A CNC hob machine is much more expensive than a mechanical hob and has capabilities that exceed what is need for gear hobbing with automatic stock divide. Existing mechanical hob machines can be retrofitted to CNC control at less expense than a new CNC machine, but are still an expensive solution for the purpose of adding an automatic stock divide feature to a hob.
The present invention provides an automatic stock divide device that can be adapted to mechanical gear hobs at significantly less cost than a CNC retrofit of the hob machine. This will allow the hob to cut or re-cut gear parts that already have teeth in them by automatically synchronizing the gear teeth with the cutter. A servomotor coupled to the differential input shaft performs the adjustment. With the workpiece and the hob cutter rotating, an input to the differential causes the workpiece to rotate relative to the hob cutter, to bring the existing teeth on the workpiece into phase with the teeth of the hob cutter. The servomotor utilizes the same differential input shaft used to make helical gears.
To accomplish the stock divide, the hob is equipped with a hob shift position sensor and a hob cutter rotary position sensor. These sensors input to a controller the lateral position of the hob cutter along the cutter axis (hob shift) as well as the rotary position of the hob cutter about the cutter axis. A stock divide sensor is mounted adjacent the worktable to determine the location of the gear teeth previously formed in the workpiece. A servomotor is mounted to the differential change gears and has a rotary position feedback to the controller as well.
The automatic stock divide procedure begins after a workpiece has been mounted to the worktable. The workpiece and the hob cutter are both rotated by the hob drive motor. The stock divide sensor inputs to the controller the workpiece teeth position while the hob shift position sensor and the hob cutter rotary position sensor input to the controller the cutter position. The controller then determines the necessary adjustment in the position of the workpiece relative to the hob cutter to bring the workpiece teeth into phase with the hob cutter. Once the calculation has been performed, the servomotor turns the feed drive input shaft to the differential, causing an adjustment in the workpiece position relative to the hob cutter to bring the workpiece gear teeth into phase with the teeth of the hob cutter.
The automatic stock divide device of the present invention can be added to an automatic loaded mechanical gear hob, allowing nonstop hobbing of near net forgings or oversized parts without the cost of a CNC retrofit of a mechanical gear hob.